The present invention relates to a method and apparatus for manufacturing seamless tubes from a hollow ingot that is introduced into a continuous-tube rolling mill.
In continuous-tube rolling mills, reshaping of a hollow ingot into a continuous bloom in accordance with the prior art occurs in six to eight roll stands, the grooves of which form a groove row for a predetermined constant bloom outlet diameter (nominal caliber). All groove rows are, in principle, so shaped that a stepwise reduction in circumference is accomplished by a simultaneous decrease in the wall from the first to the last groove.
The design of a row of grooves always relates to the smallest bloom wall thickness that is associated with a rated caliber. By predetermining a corresponding stretch limit, this being understood as the ratio of the cross section of the hollow ingot to the cross section of the bloom, there are determined the diameter of the hollow ingot, which is constant for this row of grooves, and the smallest hollow ingot wall thickness required for the production of the smallest bloom wall thickness.
In accordance with the prior art, groove rolls with stretch limits of between 3.2 and 5.5 have recently been customary.
In calculating the diameter of the hollow ingot, process-dependent entrance and exit clearance values are to be taken into account. The entrance clearance (CH) is, in this regard, the difference between the inside diameter of the hollow ingot and the diameter of the mandrel. The size of the entrance clearance is determined in accordance with two different criteria. Thus, on one hand, the entrance clearance is to be selected so large that a required exit clearance is retained at the end of the deforming process so as to avoid the bloom shrinking onto the mandrel and to assure pulling of the mandrel from the bloom. On the other hand, the size of the entrance clearance results from the necessity of introducing the mandrel before the start of rolling into the hollow ingot and positioning it. In this connection, a layer of lubricant applied to the mandrel rod must remain intact and intensive local contact between the mandrel and the inner surface of the hollow ingot must be avoided before the start of the rolling process, since heat would thereby pass from the wall of the hollow ingot to the mandrel.
Particularly in the case of longer hollow ingots, sufficient linearity cannot always be assured in actual operation, so that the greatest possible entrance clearance CH is desired for the reasons indicated. Entrance clearance values of between 6.0 and 16.0 mm are known.
A large entrance clearance, however, has the considerable disadvantage that large stretching in the first groove, which is desirable for technological reasons, is substantially limited by a necessarily widely open grooving.
It is known to persons skilled in the art that the most favorable reshaping conditions are present in the first two roll grooves. Here, with a relatively thick entrance wall, it is possible to decrease wall thickness by more than 50% with relatively low resistance to change in form. The rate of change in shape is still small in the first roll groove, as is the relative speed between mandrel and material being rolled. In the second roll groove, a stretching on the order of magnitude of 1.6 can, for example, be obtained.
Starting from these problems and the disadvantages of the prior art, an object of the invention is to create a method and apparatus with which it is possible to increase the stretch in the first roll groove in order, on one hand, to increase the total stretch limit or to relieve the central roll groove.
Another object of the invention is to retain the entrance clearance values necessary for the process within suitable limits even in the case of increased stretch.
In order to achieve the foregoing objects, one form of the invention concerns a hollow ingot that is so predeformed directly before entry into the first stand of a continuous-tube rolling mill, without reduction in wall thickness, that the clearance between a mandrel and an inner surface of the hollow ingot is greater in the region of the bottom of the groove of the rolls of the first stand than the clearance in the region of roll flanks. Accordingly, the same conditions that result, on the basis of the grooving of traditional continuous-tube rolling mills, in the following stands of these mills are obtained for the entry of the hollow ingot into the first groove of the continuous-tube rolling mill. In this way the important advantage is obtained that an increase of the stretch in the first groove becomes possible, so as to permit a reduction of the total number of roll stands. This goal can be achieved by the invention in a manner that the hollow ingot is deformed in the manner proposed, for instance ovally.
In an advantageous further form of the invention, the degree of the mentioned deformation is selected so that the inner surface of the hollow ingot lies against the mandrel in the region of the roll flanks of the first stand.
In accordance with another feature of the invention, in the case of larger entrance clearance values the invention further relates to reducing in circumference simultaneously with the deforming the hollow ingot.
An apparatus for carrying out the method of the invention is characterized by a device, arranged directly in front of the first stand of the continuous rolling mill, for deforming the hollow ingot in at least one longitudinal central plane that extends offset (by 90.degree. in the case of two-roll grooving) to the longitudinal central plane or planes of the hollow ingot which pass through the groove base of the rolls of the first roll stand of the continuous-tube train. A light-weight and cost-effective manner of construction then results if only ovalization is effected with narrow rolls of slight grooving.
If the continuous-tube rolling mill is provided with three-roll grooving, the device for deforming the hollow ingot is also equipped with three rolls.
In all cases, it is advisable to place the device for deforming the hollow ingot as close a possible to the first roll stand so that the hollow ingot is ovalized only shortly before the start of the rolling, and that the advantages of a large entrance clearance are retained. Since the device is embodied as a drive apparatus, the condition exists of adjusting the speed of the device to the rolling conditions, the force and power requirements of the device being advantageously correspondingly small since there is no reduction in the wall thickness.